1. Field of the Invention
This invention relates generally to the field of crystal controlled oscillator circuits, and more particularly to a multiple bandwidth crystal controlled oscillator circuit providing rapid oscillator start-up and low operating current drain.
2. Description of the Prior Art
Crystal controlled oscillators are widely used in communication receivers to establish the operating frequency or channel on which the communication receiver is to operate. Crystal controlled oscillators have in the past been designed to provide rapid start-up times, such as required in a communication receiver having battery saver features. Such crystal controlled oscillators were generally operated at relatively high current drains, which were required to insure rapid oscillator start-up times so that the communication receiver would be capable of receiving transmitted information in relatively short periods of time. As a result, the current drain of the oscillator circuit was typically a significant portion of the communication receiver current drain, in some instances being as much as one-third of the receiver-on current drain. The ultimate battery life provided in the prior art communication receiver was primarily determined by the receiver-on current drain, and as indicated this was highly influenced by the requirement of high operating currents to insure crystal oscillator start-up in a minimum amount of time.
Synthesized communication receivers also utilized crystal controlled oscillators to provide the reference frequency for the frequency synthesizer. The time required for the receiver to receive transmitted information was determined both by the time required to effect reference oscillator start-up and the time required to stabilize the frequency synthesizer output. In order to achieve rapid frequency synthesizer start-up times, the reference oscillator start-up time had to be minimized, generally at the expense of increased current drain of the crystal oscillator. The ultimate battery life provided in a synthesized communication receiver was consequently determined not only by the receiver current drain, as in a non-synthesized communication receiver, but also by the synthesizer current drain and the increased current drain added for reference oscillator start-up.
A typical prior art crystal controlled oscillator 100 is shown in FIG. 1. When implemented using CMOS integrated circuit technology, the circuit forms a CMOS Pierce oscillator which is described in U.S. Pat. No. 3,676,801 to Musa which is assigned to the assignee of the present invention. While such an oscillator is suitable as a reference oscillator for microprocessors and microcomputers, the problem of slow start-up times, unless operated at high current levels, limits the use as a reference oscillator in a synthesized communication receiver employing battery saving techniques, unless high current drains are utilized to provide the necessary rapid oscillator start-up times.